The benthos as the basis of vendace, Coregonus albula, and perch, Perca fluviatilis, diets in an oligotrophic sub-Arctic lake

Many small lakes in northern high latitudes are oligotrophic, with very low planktonic primary production, so that benthos may be the important source of carbon for planktonic and benthic fish. To clarify this question, the stomach content analysis in fish and the stable isotopes of carbon (delta C-13) and nitrogen (delta N-15) in pelagic and benthic organisms were conducted in a sub-Arctic lake (Lake Krivoe, White Sea coast, northern Karelia, Russia). We expected to recognize the main food items, their contribution to the diet of dominating adult fish: perch, Perca fluviatilis, and vendace, Coregonus albula, and trophic links between members of lake food webs. Samples were collected in June-October 2009 (ice-free period), from littoral, profundal, and pelagial habitats. Stable isotope analysis allocated four trophic levels (TL) in lake food webs. The large perch (> 180 mm) were found as piscivorous (top) predator (TL >4), while vendace and smaller perch as omnivorous predators (TL 3.7-3.9). Stable isotope mixing model (IsoSource, delta C-13 and delta N-15) estimated low proportion (1-5%) for crustacean zooplankton in the diet of both fish species that allocated definitively to benthic littoral food webs. Great part of energy sources for fish derived from benthic/littoral compartments, including 67-75% from benthic amphipods. Our study also provides evidences for size-dependent (ontogenetic) dietary shifts in the foraging strategies for perch and for amphipods. A great importance of omnivory by consumers was revealed for lake food webs.